Amplifier



1954 N. KORMAN 2,686,232

AMPLIFIER Filed Sept. so, 1948 mil 3 n 11 en tor MTHHMEL l. libnmazvPatented Aug. 10, 1954 AMPLIFIER Merchantville, N. J., as-

Nathaniel I. Korman, signor to Radio Corp poration of Delaware orationof America, a cor- Application September 30, 1948, Serial No. 52,065

3 Claims. 1

This invention relates to high frequency signal amplification systems,particularly to those systems in which electron discharge tubes areconnected to provide a high gain over a frequency band wide enough tocarry a good video signal.

For the amplification of wide-band high frequency signals inelectron-discharge tube amplifier stages, a relatively high degree ofisolation is needed between the input and output. With even a smallamount of capacitive coupling between them, difficulties arise becauseof the undesired feedback of signals. from the output to the input. Theinherently low outputto-input capacitance of the pentode type ofelectron-discharge tubes renders these tubes suitable for thisoperation. However the larger number of electron-discharge electrodes inpentodes, introduces more undesired noise into the signals beingamplified.

It has accordingly been suggested by Sziklai and Schroeder in theirarticle in the October 1945 issue of the Proceedings of the I. R. E.,pages 701-709, to couple together two electrondischarge triodes so thatthe incoming signals are supplied to one section and the amplifiedoutput taken from the other section thereby reducing the output-to-inputfeedback capacitance. Also by operating one of the grids at groundpotential, a sufficiently high isolation is provided to make practicalhigh frequency wideband amplifiers of low noise characteristics. Howeverthis has heretofore been possible only at the expense of a reduction inavailable gain.

Among the objects of the present invention is the provision of novelamplification systems which furnish the desired wide-band high gaincharacteristics with electron discharge tube units having a minimumnumber of separately connected electrodes.

Further objects of the invention includes the provision of twin-triodetypes of electron-discharge amplification systems having a gaincomparable to the best available pentode amplifiers, but having theadvantage of introducing less noise into the signal being amplified.

The above as well as other objects of the invention will be bestunderstood from the following description of exemplifications thereof,reference being had to the accompanying drawings wherein:

Fig. 1 is a circuit diagram of one form of amplification systemembodying the invention; and

Fig. 2 is a circuit diagram of a different modification of theinvention.

According to the invention, the system utilizes a pair ofcathode-coupled cascaded triode electron-discharge amplificationsections in which the signals are introduced into the first section,amplified and passed to the second section which is of the grounded-gridtype, and there further amplified. The signal input is connecteddirectly between the grid and cathode of the first sec tion to providewide-band amplification with an overall gain at least about as high asis possible in a conventional pentode section. The first section ispreferably connected to overcome the gain-diminishing efiect ofinterelectrode capacitances by tuning out or otherwise decreasing theeffect of such capacitances.

Fig. 1 shows one form of the invention. A first electron-dischargesection it and a second electron-discharge section 28, each comprises anelectron-emissive electrode or cathode, ll, 2!, an electron collectingelectrode or plate I 2, 22, and a control electrode or grid [3, 23 forcontrolling the passage of electrons from each cathode to its plate inaccordance with the instantaneous potential of each grid with respect toits cathode, as is well known. The electron discharge is energized bythe D.-C. plate or anode current source 38 the positive terminal (B+) ofwhich is connected to the plates or electron collecting electrodes I2,22 and the negative terminal to the cathodes ll, 2! as by means of theground connections shown. Where the signal impedance of the source 30 isnot low enough a by-pass condenser BI is connected across its outputterminals to provide low impedance for signal voltages.

, Incoming signals are supplied to the first section l6 by an inputcircuit M connected directly between the grid I3 and cathode H. Theinput circuit is shown as an inductance l5 and a resistance It inseries, connected to the grid it through a D.-C. blocking capacitor H.To provide the desired band-pass response the input circuit is adjustedto form a damped parallel circuit, as by tuning the inductance with asuitable capacitance tothe mid-frequency of the band. The tuningcapacitance, indicated by the dash lines I9, may be provided in anysuitable manner such as by utilizing the grid-to-cathode capacitance oftube section [6 and/or the distributed capacitance of the inductance l5where the inductance is lumped in the form of turns of a conductor, orby the incorporation of an actual capacitor. Damping of the circuit,shown as provided by the series resistance It, may also be accomplishedby connecting a resistor in parallel with the tuned circuit elements.

The instantaneous signal voltages appearing between grid 53 and itscathode ll cause corresponding variations in the electron-discharge orplate current through section it. As a result similar signal variationsappear across the cathode loading impedance 32 shown in the form of aninductance element 34, together with a resistor 35, establishing aseries cathode return circuit for both cathodes H and 21 to the negativeor low voltage side of the energizing source 39. The cathode load 32 mayalso be arranged as a damped parallel resonant circuit as by tuning theinductance with the stray capacitance between cathode and ground,represented at 3?, 'th its own distributed capacitance if it is in theform of lumped turns of a conductor. Both cathodes l l and 2 i areconnected together so that the load 32 is common to the cathodes or bothtriode sections It, 29. This so-called cathode coupling causes thesignals put out by the 1, to appear between the ground first section iconductor and the cathode 2i. By holding the grid 23 or the secondsection 26 at ground potential with respect to the signal frequencies,the coupled signals appear between cathode 2i and grid 23 and effect acorresponding control action on the electron flow in the second section.A load circuit 24 inserted in the connection to plate 22 accordinglydevelops voltage changes which are essentially amplifications of theinput signal voltages. The plate load circuit 24 may be generallysimilar to the other loading circuits I4, 32 and may consist of acorresponding damped resonan circuit includin inductance 25 andresistance 28. To keep any signals from bein developed in the energizingmeans 30, its terminals may be by-passed by a capacitance indicated at3!.

Grounding of the grid 23 for signal voltages, may be provided as by asimple ground connection, or by the grounded by-pass capacitor liil asshown. The use of a by-pass capacitor 46 as in Fig. 1 enables a D.-C.bias voltage to be applied to the grid 23 with respect to its cathode,for controlling the gain of the section in any welllrnown manner. Asshown in this figure, a source of bias voltage 42 is employed. Thissource may conveniently be a network which derives a D.-C.

voltage controlled in any conventional manner (not shown) by theamplitude of the signal received or delivered by the stage or by any ofa plurality of prior or subsequent cascaded stages. The voltage ofsource 42 is added algebraically to any D. C. voltage between cathodeand ground produced by the passage of plate current through the cathoderesistance, to determine the actual grid-to-cathode bias.

An important advantage of the invention is the direct connection of thesignal input circuit to the grid and cathode of the firstelectron-discharge tube unit. In this manner incoming signals aredirectly impressed between these electrodes so that no attenuation isproduced by intervenin circuit elements. Furthermore inasmuch as thecathode load 32 develops signal voltages which cause the cathode H tofollow the voltage changes of grid l3, there is an inherent degenerationof any incoming signals which may be applied between the grid of thefirst unit It and ground. In the construction of Fig. 1 this is apparentfrom the fact that such cathode-toground voltages are in phase with, andare subtracted from, the grid-to-ground voltages to determine thegrid-to-cathode or control voltage.

However this degeneration is a minimum when the desired incoming signalsare directly applied between grid and cathode as shown. With thisarrangement voltage changes of the cathode H with respect to ground donot appreciably affect the grid-cathode potentials.

Due to the fact that the grid and cathode of section it are bothfloating above ground potential, the inherent gride-plate interelectrodecapacitance of section l0, represented by the dashline capacitor 45, mayhave an appreciable gaindiminishing effect. Since plate i2 is groundedfor signal frequencies, as by 13+ by-pass condenser Sl, this plate iseffectively connected to the grounded side of the cathode load 32.Without the capacitance d4, the cathode load 32 has only one connectionto the input circuit at cathode H. Capacitance 44 may be considered ascompleting a feedback connection from the cathode load to the inputreturning signals opposite in phase to the incoming signals. The abovegaindiminishing effect may be reduced by decreasing signal transferthrough the grid-plate capacitance.

In the form of the invention shown in Fig. l the effect of thegrid-to-plate capacitance circuit is reduced by raising the circuitimpedance. This is accomplished by including the capacitance withinductive impedance in a parallel-resonant circuit tuned to the signalsbein amplified, as shown in Nichols 1,325,879 December 23, 1919. Theadded inductance may be connected through a suitable blocking capacitordirectly between grid is and plate l2 or between grid l3 and any othermember which is efiectively at ground otential with respect to thesignal frequencies. As shown, a tuning inductance 46 is connectedbetween grid l3 and grounded-grid 23. The inductive impedance connectionmay include resistance either in parallel or in series, as indicated at48 to broaden the resonance. Lumped or distributed inductances may beutilized.

A feature of the inductance connection to grid 23 is that thisinductance simultaneously establishes a return circuit for grid l3thereby providing the connection necessary for the maintenance ofsuitable bias between grid 13 and its cathode. If desired, the biassupply 42 may be omitted so that only the voltage drop across thecathode return circuit 32 supplies the grid biasing of the stages.By-pass capacitor 46 should also be omitted in this variation inasmuchas a direct grid return to ground is all that is required.

In actual use the system of Fig. 1, with electron-discharge tubes of the6J6 type in which two triodes corresponding to sections it and 25 aremounted in a single tube envelope, has been found to provide an overallgain about 20 percent higher than the highest gain presently availablewith a pentode high-frequency wide-band stage of amplification.Furthermore the undesired noise injection of the amplifier system of theinvention is much lower than that of pentodes apparently because of theelemination of the separately connected extra electron dischargeelectrodes.

The input circuit it may have its inductance l5 inductively coupled witha signal supply, as by connecting the inductance it as the secondary ofa coupling transformer 9. The amplified output may be derived from theplate-loadin circuit 24 as by inductively coupling the plate inductance26 with a second inductance 29 to form an output transformer.

Fig. 2 shows a modified form of the invention.

This form is generally similar to that shown in Fig. 1. Two triodeelectron-discharge sections 2-H, 2--2Ei are her shown as incorporated inseparate tubes with their cathodes coupled through a common cathodereturn circuit 232. Grid 225 of the second stage 2-2li is directlygrounded. Input circuit 2M, is connected directly between grid 2l3 andcathode 2-4! to make the fullest use of incoming signals and providemaximum gain.

Fig. 2 also shows another technique for decreasing the gain-diminishingeffect of the inherent grid-plate capacitance. The input circult 2-l iincludes inductance 2-I 5 across which incoming signals are supplied andthis inductance is provided with a grounded tap 2-48. The ground tap canbe considered as effectively tying th input circuit to ground andthereby reduces the capacitance to ground which it would otherwise haveif completely floating, as in the construction of 1. By properlylocating the D.-C. grid blocking capacitor 2il, the ground tap 2-48 mayalso be arranged to establish a return and/or bias connection for grid2-l3. This is accomplished by inserting the blocking capacitor 2-5? inany desired portion of the cathode connection of the inductance 2-!5,between this inductance and a loading resistance 2-!6 for example.

The inductance 2-l5 may be arranged to receive incoming signals byinductive or magnetic coupled with another inductance 25t carrying thedesir d incoming signals. The amplified output may be similarly takenfrom a plate loading circuit 2-43 in the plate connection of the secondsection 2-2i3, or it may be derived in any other suitable manner such asby capacitive coupling to a portion of the plate load. A take-offconnection 252 at the high-signal-potential side of the plate load isespecially suitable.

A particular advantage of the construction of Fig. 2 is its combinationwith an antenna of the balanced type such as the dipole 25l. Not only isthe noise level kept low by reason of the fact that triodeelectron-discharge sections are used in the first amplification stagewhere the signal level is low and the noise injection is critical, butin addition the construction shown functions to diminish the noise leveldue to undesired pickup in the antenna. These signals to which thebalanced antenna is not resonant are developed essentially between theantenna and ground and, when induced in the secondary 2-45 of the inputtransformer, return to ground through the tap 2-!8. Such transfer ofundesired signals causes them to produce signal currents in inductance2-45 which pass in opposite directions from its ends to its ground tapand can be made to balance against each other so that they cancel eachother and are substantially absent from the amplified output.

The ground tap 2-i8 may be at the mid-point of inductance 2i5 or it maybe displaced somewhat in either direction from this point. Adjustabilitymay be incorporated for setting in the position providing the desiredoverall performance. some resistance or inductance included in theground tap return improves the overall gain.

The various load circuits may have their damping resistors connected inparallel or in series with the resonant circuits. Additionallycapacitances may be deliberately inserted in these circuits for tuningthe inductances, as where the signals to be amplified do not cover verywide bands and the maximum L/C' ratio is not required. For theamplification of signals in the television hands, it is generallypreferred to have minimum capacitance, but where capacitance is to beadded, it may very simply be effected by merely bringing into closeproximity those conductors between which additional capacitance isdesired.

The features of the invention are also suitable for inclusion in atwo-Way amplifier arrangement where additional signals are supplied togrid 23 or 2-23 with respect to its cathode or with respect to ground.As described in the above-mentioned Sziklai and Schroeder article, ifthe additional signals are far enough removed from the principal signalband, they are also amplified and delivered to the plate of the firstsection from which they may be recovered by inserting a load circuit inthe connection to this plate. The input and output loads of each of thesignals are arranged to be or negligible impedance to the other signal.The additional signals may also be arranged to modulate the gain of thesecond section to thereby cause heterodyning or mixing of the signalsand supply a beat signal output for intermediate frequency channels.

The gain-diminishing effect of the undesired grid-plate capacitance ofthe first amplification section may be counteracted by other techniquessuch as neutralization of this capacitance.

While several exemplifications of the invention have been indicated anddescribed above, it will be apparent to those skilled in the art thatother modifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:

1. In a high frequency amplifier system for amplifying a broad band ofhigh frequency signals, an amplifying device having a control electrode,an electron collecting electrode and an electron emissive electrode, anungrounded input circuit responsive to said band connected between saidcontrol electrode and said emissive electrode, a load impedance circuitresponsive to said band connected between said emissive electrode and apoint of ground potential, said collecting electrode being energized bya source of direct-current potential and effectively grounded for saidband of signals, said device being characterized by a substantialcapacitance between said collecting and control electrodes thatcompletes a degenerative feedback path for said band between said loadimpedance circuit and said input circuit with a resultantgain-diminishing effect, impedance means comprising an inductorconnected eifectively between said input electrode and said point ofground potential for substantially overcoming said gain-diminishingefiect of said capacitance, said impedance means being connectedeffectively in shunt relation with said capacitance and of such valuethat it resonates broadly with said capacitance to said band, and asecond amplir'ymg device having control, collecting and emissiveelectrodes coupled to said first named device, said emissive electrodesof said devices being directly connected together and said controlelectrodes of said devices being conductively connected through saidinductor, said second named control electrode being efiectively groundedfor said band of signals.

2. The invention as set forth in claim 1 wherein a bias source isconnected between ground and said second-named control electrode forbiasing same and, through said inductor, said first-named controlelectrode.

3. In a high frequency amplifier system for amplifying a band ofsignals, first and second amplifying devices each having a controlelectrode, an electron collecting electrode and an emissive electrode,an input circuit responsive to said band connected between said inputelectrode and said emissive electrode of said first device, a loadimpedance circuit responsive to said band connected between saidemissive electrode of said first device and a point of ground potential,said collecting electrode of said first device being energized by asource of direct current potential and effectively grounded for saidband of signals, said first device being characterized by a substantialcapacitance between said collecting and control electrodes thatcompletes a degenerative feedback path between said load impedancecircuit and said input circuit with a resultant gaindiminishing effect,said second device having its emissive electrode direct-connected tosaid emissive electrode of said first device, its control electrodeeffectively grounded for said band of sig nals and an output circuitconnected to its collecting electrode, and inductive impedance meansconnected between said control electrodes of said devices and being ofsuch value as to resonate with said capacitance to said band forsubstantially nullifying the gain diminishing effect of saidcapacitance.

References Cited in the file of this patent UNITED STATES PATENTS NumberNumber Name Date Hathaway et a1. Sept. 16, 1941 Crosby Mar. 17, 1942Koch May 11, 1943 Bedford Apr. 29, 1947 Atkins May 11, 1943 SchroederFeb. 8, 1949 Adams Apr. 24, 1951 FOREIGN PATENTS Country Date GreatBritain Sept. 20, 1938

